In this answer How are condensation cones created by supersonic airplanes? it is said that the condensation cones appear only when the airplane flies at speeds below Mach 1. I am wandering why does it disappear above Mach 1, and does the condensation cone appear when flying exactly at Mach 1?
$\begingroup$ Exactly Mach 1 isn’t really a thing, as the airflow will accelerate to some degree as it passes over each aerodynamic surface. $\endgroup$– FrogSep 10, 2022 at 9:06
Condensation occurs when the air accelerates, which is accompanied by drop in pressure and temperature that can push it below the dew point if humidity is high.
As explained in What is compressibility drag?, at subsonic speeds the air accelerates and expands to make way for the aircraft, but it compresses and slows down at supersonic speeds. So the condensation only occurs at subsonic speeds.
Now the condensation can be seen under various circumstances when the humidity is high: over the wings of landing planes, over the wing roots of planes pulling high G, in the core of the wake vortices, and also inside a lambda shock at transsonic speeds, which is the condensation cone.
The transsonic condensation cone is particularly noticeable because, as explained in What is compressibility drag?, near the speed of sound the decrease in pressure leads to decrease in density, so the air must accelerate to a much bigger distance from the aircraft to make a room for it, making the condensation cone much bigger than the other cases of condensation around the aircraft.